1. Field of the Invention
The present invention relates to a method and an apparatus for producing a Group III nitride compound semiconductor. More particularly, the invention relates to a method for producing a thick-film Group III nitride compound semiconductor or for producing a Group III nitride compound semiconductor having a thickness which allows the semiconductor to be handled as an epitaxial growth substrate.
As used herein, the term “Group III nitride compound semiconductor” refers to a semiconductor having arbitrary compound crystal proportions and represented by AlxGayIn1-x-yN (0≦x≦1; 0≦y≦1; 0≦x+y≦1), and encompasses 2-component semiconductors such as AlN, GaN, and InN; 3-component semiconductors such as AlxGa1-xN, AlxIn1-xN, and GaxIn1-xN (in each case, 0<x<1); and 4-component semiconductors represented by AlxGayIn1-x-yN (0<x<1; 0<y<1; 0<x+y<1). The “Group III nitride compound semiconductor” also encompasses semiconductors which contain a Group III element such as boron (B) or thallium (Tl) and in which nitrogen atoms are partially substituted by Group V elements such as phosphorus (P) and arsenic (As). Unless otherwise specified, in the present specification the term “Group III nitride compound semiconductor” also encompasses such semiconductors which are doped with an impurity for determining a conduction type of p or n.
2. Description of the Related Art
When Group III nitride compound semiconductors such as those represented by AlxGayIn1-x-yN (0≦x≦1; 0≦y≦1; 0≦x+y≦1) are produced through epitaxial growth, a substrate for growth is required. However, Group III nitride compound semiconductor substrates for use in epitaxial growth having a manageable thickness are not commercially available. Therefore, substrates produced from dissimilar materials other than Group III nitride compound semiconductors, such as a sapphire substrate, a silicon carbide (SiC) substrate, and a silicon (Si) substrate, have been employed instead.
However, the dissimilar substrates have lattice constants which differ considerably from those of Group III nitride compound semiconductors. Thus, generally, a so-called buffer layer is grown on the dissimilar substrate in advance, and a Group III nitride compound semiconductor is epitaxially grown on the buffer layer. Even when this approach is employed, large thermal stress is generated by difference in thermal expansion coefficient between the substrate and the Group III nitride compound semiconductor during cooling to room temperature after epitaxial growth conducted at a temperature as high as 1,000° C. or even higher. In other words, even though epitaxial growth can be successfully performed at high temperature, a significant difference in thermal expansion coefficient between the substrate and the Group III nitride compound semiconductor induces a large number of crystal defects and cracks in the substrate and the Group III nitride compound semiconductor during cooling to room temperature.
Specifically, approximate linear expansion coefficients (at about room temperature, along the a-axis) of gallium nitride (GaN), aluminum nitride (AlN), sapphire (α-Al2O3), and silicon (Si) are 5.6×10−6/K, 4.2×10−6/K, 7.5×10−6/K, and 3.6×10−6/K, respectively. Accordingly, when thick-film GaN is formed on a sapphire (α-Al2O3) substrate or a silicon (Si) substrate, followed by cooling by 1,000 K (or ° C.), a difference in shrinkage as large as 0.2% arises along the a-axis. Particularly when a GaN layer having a thickness of several tens of μm or more is formed on a sapphire (α-Al2O3) substrate or a silicon (Si) substrate, cracks are inevitably generated in the formed GaN layer and in the sapphire (α-Al2O3) substrate or the silicon (Si) substrate, due to thermal stress during cooling.
From another viewpoint, it has been considered almost essential that production, through epitaxial growth, of a Group III nitride compound semiconductor, particularly gallium nitride (GaN), of an excellent single-crystalline state requires a high temperature of 1,000° C. or higher.